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Geological Society of America Special Paper 399

2006

Evolution and importance ofwetiands in earth history

Stephen F. Greb

Kentucky Geological Surrey, 228 MMRB University of Kentucky, Lexington, Kentucky 40506, USA William A. DiMichele

Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D. C. 20560, USA Robert A. Gastaldo

Department of Geology, Colby College, Waterville, Maine 04901-8858, USA

ABSTRACT

The fossil record of wetlands documents unique and long-persistent floras and faunas with wetland habitats spawning or at least preserving novel evolutionary characteristics and, at other times, acting as refugia. In addition, there has been an evolution of wetland types since their appearance in the Paleozoic. The first land plants, beginning in the Late Ordovician or Early Silurian, were obligate dwellers of wet substrates. As land plants evolved and diversified, different wetland types began to appear. The first marshes developed in the mid-Devonian, and forest swamps originated in the Late Devonian. Adaptations to low-oxygen, low-nutrient conditions allowed for the evolution of fens (peat marshes) and forest mires (peat forests) in the Late Devonian. The differentiation of wetland habitats created varied niches that influenced the terrestrialization of arthropods in the Silurian and the terrestrialization of tetrapods in the Devonian (and later), and dramatically altered the way sedimentological, hydrological, and various biogeochemical cycles operated globally.

Widespread peatlands evolved in the Carboniferous, with the earliest ombrotrophic tropical mires arising by the early Late Carboniferous. Carboniferous wetland- plant communities were complex, and although the taxonomic composition of these wetlands was vastly different from those of the Mesozoic and Cenozoic, these communities were essentially structurally, and probably dynamically, modern. By the Late Permian, the spread of the Glossopteris flora and its adaptations to more temperate or cooler climates allowed the development of mires at higher latitudes, where peats are most common today. Although widespread at the end of the Paleozoic, peat-forming wetlands virtually disappeared following the end-Permian extinction.

The initial associations of crocodylomorphs, mammals, and birds with wetlands are well recorded in the Mesozoic. The radiation of Isoetales in the Early Triassic may have included a submerged lifestyle and hence, the expansion of aquatic wetlands.

The evolution of heterosporous ferns introduced a floating vascular habit to aquatic wetlands. The evolution of angiosperms in the Cretaceous led to further expansion of aquatic species and the first true mangroves. Increasing diversification of angiosperms in the Tertiary led to increased floral partitioning in wetlands and a wide

Greb, S.F., DiMichele, W.A., and Gastaldo, R.A., 2006, Evolution and importance of wetlands in earth history, in Greb, S.F., and DiMichele, W.A., Wetlands through time: Geological Society of America Special Paper 399, p. 1^0, doi: 10.1130/2006.2399(01). For permission to copy, contact [email protected].

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S.F. Greb, W.A. DiMichele, and R.A. Gastaldo

variety of specialized wetland subcommunities. During the Tertiary, the spread of grasses, rushes, and sedges into wetlands allowed for the evolution of freshwater and salt-water reed marshes. Additionally, the spread of Sphagnum sp. in the Cenozoic allowed bryophytes, an ancient wetland clade, to dominate high-latitude mires, creat- ing some of the most widespread mires of all time. Recognition of the evolution of wet- land types and inherent framework positions and niches of both the flora and fauna is critical to understanding both the evolution of wetland functions and food webs and the paleoecology of surrounding ecotones, and is necessary if meaningful analogues are to be made with extant wetland habitats.

Keywords: paleobotany, paleoecology, paleoflora, earth history, wetlands, coal, swamp, mire, marsh, fen, bog.

INTRODUCTION

Modem wetlands are characterized by water at or near the soil surface for some part of the year, soils that are influenced by water saturation all or part of the year, and plants that are adapted to living in conditions of water saturation all or part of the year (National Research Council, 1995; Keddy, 2000; Mitsch and Gosselink, 2000). Many wetlands occupy lowlands and natural depressions, so have a relatively high preservational potential. It is not surprising, then, that a large part of the fossil record of terrestrial flora and fauna (especially in the Paleozoic) is found within wetlands or wetland-associated habitats. These deposits provide windows into ancient biodiversity, but frequently represent a mix of allochthonous and autochthonous material from different ecosystems. In order to examine the importance of wetlands through time, it is important to recognize that there are many different types of wetlands and wetland functions, and that both have changed through time.

a starting point for discussion of paleowetlands. The terms are similar to wetland classes in the hierarchical Canadian wetland system (Zoltai and PoUett, 1983). Such hierarchical classifications are commonly used to describe modern wetlands. In the context of characterizing paleowetlands on the basis of standard- ized wetland classifications, additional modifiers such as marine, estuarine, riverine, palustrine, and lacustrine are used where appropriate for comparison to the U.S. Fish and Wildlife wetland classification (Cowardin et al., 1979). Modifiers such as marine/

coastal and inland are used where appropriate to indicate relative equivalence to wetlands in the Ramsar Convention classification.

Modifiers also are used to describe wetland forms, types, and varieties as described in the Canadian system or to describe form, hydrology, or nutrient status for peat-producing wetlands (Gore, 1983; Moore, 1989,1995; Mitsch and Gosselink, 2000). Each of the modern systems is designed for different purposes, so varied modifiers from each are used to describe clearly paleowetlands discussed in this report.

Types of Wetlands Wetland Functions

Holocene wetlands have been classified variously over the past several decades, with workers on different continents and in different hemispheres using a range of terms to classify wetlands on the basis of hydrology, geography, and flora, among other criteria. Unfortunately, variable definitions and terminology can lead to uncertain or mistaken use of analogues when interpreting the paleoecology of ancient wetlands. For the purposes of this investigation, we use the following general terminology adapted from Keddy (2000): aquatic (or shallow water) wetland for wetlands dominated by submerged vegetation under continually inundated conditions; marsh for wetlands dominated by herbaceous, emergent vegetation rooted in mineral (non-peat) substrates; swamp for forested wetlands on mineral (non-peat) substrates; fen or nonforested mire for wetlands dominated by herbaceous or shrub vegetation on peat substrates. Because there is considerable variability in the use of the term bog (Keddy, 2000; Mitsch and Gosselink, 2000), the i&rm forest mire is used herein for forested peats. These general terms can have a wide array of meanings (Mitsch and Gosselink, 2000) but serve as

Modem wetlands provide many critical functions in global ecology, including providing habitat and food for diverse species, and aiding in groundwater recharge and water retention and detention, which allows for maintenance of high water tables in wetlands as well as reduced flooding in adjacent ecosystems.

They also provide erosion and sedimentation controls between adjacent ecosystems, improve water quality through filtering sediment and metals from groundwater, and cycle nutrients to terrestrial and aqueous environments within the wetland and between ecotones (National Research Council, 1995; Keddy, 2000; Mitsch and Gosselink, 2000). Wetlands are also important global sources, sinks, and transformers of various elements in the earth's various biogeochemical cycles (National Research Coun- cil, 1995; Keddy, 2000; Mitsch and Gosselink, 2000). As fuU or part-time habitats, they function as a significant repository of the world's biodiversity (Bacon, 1997; Keddy, 2000; Mitsch and Gosselink, 2000). These functions are important not only within the wetlands themselves, but also to surrounding ecosystems. Not all functions are equally distributed through the different types

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Evolution and importance of wetlands in earth history of wetlands, and many are influenced by particular floras and

faunas. Because the floras, faunas, and types of wetlands have evolved through time, wetland functions have changed through time, as well.

Wetland Niches and Associations

The variety of organisms adapted to various wetland habitats is large and includes all major groups of animals and plants (Bacon, 1997). Herein, we examine the evolution of some common wetland faunal and floral associations. Changes in wetland niches and associations have occurred as the various adaptive strategies of plants and animals have evolved. In some cases, the extant wetland biota lives under conditions similar to those of ancient wetland plants and animals. In others, framework positions or habitats have evolved through specialization, resulting in new wetland types and functions.

Analyses of Paleowetlands

There has been extensive research on ancient wetlands, mostly centered on coals because of their economic value. Sev- eral papers have specifically examined floral change in coal-forming floras through time, sometimes concentrating on a particular era (e.g.. Shearer et al., 1995) or region (e.g.. Cross and Phillips, 1990). Some reports also have used various aspects of coal distribution through time to further understanding of global changes in tectonics, climate, and eustasy (e.g., Scotese, 2001). In terms of wetlands, such reviews tend to be focused on peat-forming mires, which represent a subset of wetland types. In fact, coals are often generalized as representing wetlands, which has the unfortunate result of marginalizing the significance of non-coal facies as wetlands of importance. The understanding that coal floras and "roof shale floras represent different types of wetlands (e.g., Gastaldo, 1987), emphasizes that non-peat producing wetlands are well represented in the fossil record. In some cases, at different times in earth history, these non-peat producing wetlands may have been more important, in terms of their functions and influences on ecotones, than mires.

Numerous botanical and biogeographical studies have dem- onstrated how changing climate or timing of tectonic movements changed the composition of Tertiary floras (including wetland inhabitants) in different areas (e.g., Aaron et al., 1999). In terms of climate, it also is important to understand the bias imposed by the present global climate on wetlands and wetland floras.

Pfefferkom (1995) noted the need for a reorientation of a per- ceived north-temperate perspective and search strategy for interpreting ancient mire ecosystems. Likewise, Collinson and Scott (1987) pointed out the importance of understanding differences in a flora through time when attempting to reconstruct ancient mires. Similarly, it is important to understand changes in specific types of wetland ecosystems. Extant floras and faunas occupy specific niches in different types of wetlands, some of which entail unique physiological adaptations and ecological interac-

tions. These adaptations have changed through time. In some cases, novel floral adaptations have led to new types of wetlands, wetland functions, and wetland faunal niches.

Purpose

Herein the evolution of wetland ecosystems through time is analyzed. We focus on the development of new and changing wetland ecosystems, which accompanied the evolution of the terrestrial flora and, in turn, influenced the evolution of numerous animal groups through the evolution of new niche space, food sources, and habitat. The unusual chemistry and sedimentology of wetland systems resulted in a wide variety of traps in which both fauna and flora are preserved. Significant wetland fossil sites that offer snapshots of ancient biodiversity and paleoecology are also highlighted in order to illustrate the importance of wetland ecosystems to our understanding of earth history. Likewise, we examine the origins and changing influences of specific wetland functions through time to illustrate the potential importance of wetland ecosystems on neighboring ecosystems and in some cases, global paleoecology. The fossil record is our best tool for understanding how changes in wetland distribution, type, niches, and functions influence non-wetland ecosystems, which is particularly important when trying to understand potential long-term natural and anthropogenic influences on global ecology.

ORDOVICIAN-SILURIAN Prevascular Wetlands

The origin of land plants appears to have occurred in the Late Ordovician to Middle Silurian, involving pre-tracheophyte, embrophytic or bryophytic (moss, lichen) plants that were obligate dwellers of wet substrates (Gray et al., 1982; Gensel and Andrews, 1984; Taylor, 1988; Stewart and RothweU, 1993;

Tomescu and RothweU, this volume). Whether these prevascular plant-vegetated substrates can be considered wetlands depends on the definition used, and Retallack (1992) has proposed a separate terminology for the associated paleosols. If a "wetland" can be defined simply as consisting of vegetation on a wet substrate, then this habitat has its origin with these vascular precursors.

Using the classification scheme of Cowardin et al. (1979), these habitats come closest to representing fluvial and paludal moss- lichen wetlands in which mosses or lichens cover a saturated mineral substrate, other than rock, and dominate the vegetation.

They obviously would have differed significantly from extant moss-lichen wetlands in not being associated with any vegetation of taller stature. Pre-Devonian moss-like wetlands also were non-peat-accumulating and therefore would not be termed bogs or fens, nor would they be expected to have similar ecology and functions to those of extant Sphagnum moss-dominated mires.

If wetlands are defined by the presence of hydrophytic vascular plants, then, by definition, wetland origins are tied to the origin of vascular plants in the Middle Silurian.

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S.F. Greb, W.A. DiMichele, and R.A. Gastaldo

SILURIAN

The Oldest Vascular Plants in Wetlands

By many accounts, Cooksonia (Wenlockian) is considered the oldest vascular plant (Edwards, 1980; Edwards and Fanning, 1985J. Cooksonia is a rhyniophyte, a group of small, simple, stick-like vascular plants. It is found mostly in autochthonous deposits associated with fluvial sandstones and floodplains.

Edwards (1980) inferred Cooksonia habitats along large rivers, which might indicate inland fluvial wetlands according to the Cowardin et al. (1979) classification. The term "riparian wetland," which describes wetlands and associated upstream areas influenced by the river, also would apply, although there would be few functional similarities to extant riparian settings because of the small stature of these rhyniophytes. Cooksonia only grew to a few centimeters, so was moss-like in stature. The term "marsh"

(often used in descriptions of these wetlands) is functionally

problematic in its application to Cooksonia-domm&itd wetlands.

Marshlands generally are considered to be dominated by deeply rooted herbaceous vegetation (e.g., Keddy, 2000), decimeters to meters in height (Mitsch and Gosselink, 2000; Keddy, 2000).

Pre-Late Devonian plants were mostly less than a meter in height and were not deeply rooted (Fig. 1).

Some research has inferred that simple rhyniophytoid plants, like Cooksonia, inhabited salt marshes (Jeram et al., 1990; Shear et al., 1989). Modern salt marshes are a special wetland type inhabited by a low diversity of plants adapted to salt stress caused by brackish to marine tidal inundation or sea spray. Late Silurian plants were simple plants lacking morphological features common in modern salt marsh plants, such as deeply buried rhizomes, salt-excluding roots (e.g., pneumato- phores), and bark or leaves that might contain salt glands, and do not appear to have any obvious adaptations to varying soil salinities. As a consequence, it is likely that early rhyniophytes grew under freshwater conditions.

Silurian Devonian Mis.

Lower Upper Lower Middle Upper Low.

Llandovery Ludlow Lochkovian

(Gedinnian) c ro c 'c

to 0) en O)

c ra

LLI

Frasnian c

<5 'c c

<u

e

C

<5 (0

•(0 c

? "moss-like" wetlands--

Origin of wetland types r

1 I Vascular plants

• swamps-

"marsh-like"—

• marshes-

shrub mires/ fens (peat marshes)—t

mm 400-

300- 200- 100- 0-

Asteroxyion Aglaophyton sawdonia \ .

(Rhynia major) \ ^ * Rhynia gwynne \

Horneophyton,. \.

Zosterophyllum ^^ ^v \ .

Cooksonia y Hln Lf

vv.YfYTT,J

& c

Q.

2 c

w fe oi Archaeopteris A

forest mires (peat swamps)

— 6m

thalloid plant 9 ground cover

Sawdonia Cooksonia Rhynia \ /

• ••...••, . . T»» »:r>f

"I \

400

Millions of years ago

440 420

Figure 1. Evolution of wetland types in the Silurian and Devonian. The heights of major floral components are shown as is the inferred depth of rooting. Heights of plants from various sources. Estimates of root depth from Algeo et al. (1995).

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Evolution and importance of wetlands in earth history

LATE SILURIAN-EARLY DEVONIAN DEVONIAN

Arthropod Terrestrialization in Wetlands The Spread of Wetlands Arthropods are the oldest terrestrial animals. Putative paleo-

sols and terrestrial arthropod trace fossils are inferred for strata as old as the Ordovician (Retallack and Feakes, 1987; Retal- lack, 2000; Shear and Selden, 2001), but the oldest undisputed terrestrial land animal, Pneumodesmus, is a millipede from the Middle Silurian of Scotland (Wilson and Anderson, 2004).

Upper Silurian terrestrial arthropods include trigonotarbids (spider-like arachnids), kampecarids (millipede-like arthropods) and fragments of possible centipedes (Jeram et al. 1990; Rolfe, 1990). Silurian arthropod terrestrialization was linked closely to vascular plant evolution in wetlands (Rolfe, 1980; Jeram et al., 1990). In fact, the transition from an aqueous to a terrestrial habit may have been aided by low-structured vegetation that created humid microclimates near the soil surface (Rolfe, 1985). Most Late Silurian and Early Devonian arthropods are found associated with freshwater marsh-like vegetation in both autochthonous and allochthonous deposits, providing the earliest evidence of habitat function in wetlands. The oldest possible insect is the fragmentary remains of Rhyniognatha, from the Lower Devonian (Pragian) Rhynie Chert (Engel and Grimaldi, 2004). The slightly younger and more complete remains of a bristletail from the Emsian (Lower Devonian) of Quebec, Can- ada, was inferred by Labandeira et al., (1988) to indicate hexapod origins in wet, marsh-like habitats. Similar deposits from the Emsian of Canada have produced millipedes, arthropleurids, and terrestrial scorpions (Shear et al., 1996). The Alken- an-der-Mosel fauna (Emsian), which includes trigonotarbids, arthropleurids, and the oldest non-scorpion arachnid (St0rmer, 1976), is preserved along with lycopsids and rhyniophytes (wetland plants) (Jeram et al., 1990; Shear and Selden, 2001). The Middle Devonian (Givettian) Gilboa fauna includes eurypterids and terrestrial arthropods, including arachnids, centipedes, a possible insect, and the oldest spider, and is in association with herbaceous lycopsids and progymnosperms (Shear et al., 1984;

Selden etal., 1991).

The spread of kampecarid arthropods (myriapods) is an example of the possible paleoecological significance of wetlands in arthropod evolution. Kampecarids were millipede-like arthropods that were restricted to freshwater aquatic or near- aquatic habitats in which they fed on plant detritus (Almond, 1985). In the Silurian, plant detritus would have been restricted in and around moss-like to marsh-like wetlands. Modern millipedes prefer moist litter horizons and dead wood as habitats, and they are critical agents for nutrient cycling in tropical wetlands and wetland forests as litter-horizon detritivores. The radiation of kampecarids and true diplopods (millipedes) into the earliest wetland communities undoubtedly contributed to increased nutrient cycling, which increased soil quality and contributed to increasingly complex food webs as the terrestrial floral and faunal radiations progressed.

Most of the Early to Middle Devonian terrestrial fossil record is confined to subtropical-to-tropical wetland habitats, with plants restricted to monotypic stands in freshwater, near-channel, deposits (Edwards, 1980; Beerbower, 1985; Edwards and Fanning, 1985; DiMichele and Hook, 1992). Hence, these assemblages mostly would be classified as paludal or riverine wetlands. Late Silurian rhyniophytes were joined by several new clades in the Early Devonian, including zosterophylls (Gedinnian) and trimerophytes (Siegenian) (Kenrick and Crane, 1997; Bateman et al., 1998), all low-stature (centimeters in height) vegetational types (Fig. 1). Lycopsids also are found in the Early Devonian (Siege- nian), and may represent an additional new clade if a Silurian age for Baragwanathia is discounted. Baragwanathia, a primitive lycopsid from Australia, originally was assigned a Late Silurian (Ludlovian) age (Lang and Cookson, 1935; Garratt et al., 1984), but this determination is controversial. Baragwanathia actually may be of Early Devonian age (Edwards et al., 1979).

All Early Devonian vascular plants were small and homo- sporous, which means that their gametophytes required water- mediated fertilization (Remy, 1982). Likewise, the small rhizoids of these rhyniophytes, trimerophytes, and zosterophylls indicate habitats characterized by nearly continuous moisture (DiMichele and Hook, 1992; Hotton et al., 2001)—in other words, moss-like to at most marsh-like wetlands, but still smaller in height than the flora that typically inhabits extant marshes (Fig. 1).

Geothermal Wetlands

By far the most famous early terrestrial biota is from the Rhynie Chert (Siegenian) of Scotland. Chert in this wetland deposit preserves the three-dimensional remains of fungi, algae, small non- vascular polysporangiophytes, a lycophyte, small vascular plants, arachnids (mites, trigonotarbids), an insect, and freshwater crustaceans (Remy and Remy, 1980; Rolfe, 1980; Trewin, 1996; Rice et al., 2002). Rhyniophytes have been interpreted as "swamp" (e.g..

Knoll, 1985), "marsh" (Trewin and Rice, 1992), and "bog" plants (Rice et al., 1995), although the terms have been applied somewhat indiscriminately. Although the term "swamp" is sometimes used informally to describe any type of wetland, formal use in several classification systems requires arborescent vegetation, which were lacking at Rhynie. "Marsh-like" rather than "marsh" might be more appropriate because of the small stature of herbaceous vegetation preserved. The term "bog" is even more problematic because bogs are peat-accumulating wetlands. Although silicified organic lami- nae have been called "peat mats" at Rhynie (Knoll, 1985), these are not thick (millimeters thick) and much thicker peats would be more typical of the modem peat-forming wetlands classified as bogs.

Recently, the cherts were shown to have been deposited in a fluvio-lacustrine setting within, or on the margin of, a hydrothermal basin (Trewin and Rice, 1992; Trewin, 1994, 1996;

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S.F. Greb, W.A. DiMichele, and R.A. Gastaldo Rice et al., 1995, 2002). In situ plant assemblages accumulated

in ambient waters of interfluves and overflow pools between hydrothermal ponds and geysers. Hence, at least some part of the Rhynie Chert biota represents inland geothermal wetlands as defined in the Ramsar classification (Fig. 2).

The association of freshwater crustaceans with the Rhynie biome is interesting because crustaceans are one of the most common groups of modem wetland-inhabiting arthropods. The Rhynie crustaceans {Lepidocaris, Castracollis) are branchiopods, similar to modem tadpole shrimp (Triops) and fairy shrimp (Artemia) (Anderson and Trewin, 2003; Payers and Trewin, 2004). Extant branchiopods are common in wet meadows (vemal ponds) where they are important parts of detritivore-based food webs. Extant wet meadows are ephemeral wetlands dominated by herbaceous grasses and shrubs (Keddy, 2000; Mitsch and Gosselink, 2000).

Crastaceans can thrive in ephemeral wetlands because of the lack of fish, which also seems to have been the case in the Rhynie eco- system. Today, wet meadows (considered by some as a subset of marshes) are dominated by angiosperms (grasses and sedges). In the middle Paleozoic, rhyniophytes may have occupied similar niches, although rhyniophytes were likely less drought resistant than the flora of wet meadows today, and the relationship between their life history pattem and seasonal drought is not understood.

The Oldest Marshes

By the Middle Devonian (Eifelian), several plant groups had evolved shrub or bush morphology (Pig. 1). The lycophyte

Asteroxylon mackiei (Emsian-Givetian) from the Rhynie Chert may have grown to heights of 50 cm (Gensel and Andrews, 1984;

Gensel, 1992). Pertica quadrifaria, a trimerophyte from the Trout Valley of Maine (United States), grew to at least a meter in height (Kasper and Andrews, 1972; Allen and Gastaldo, this volume) if not taller As such, wetlands comprised of these emergent plants formed the earliest marshes (inland shrub-dominated wetlands sensu Ramsar classification). Middle Devonian wetlands began to exhibit floral partitioning (Allen and Gastaldo, this volume), possibly in response to salinity, water chemistry, nutrients, or sedimentation and flooding (duration and periodicity of inundation). This partitioning undoubtedly involved feedback loops with stands of vegetation also influencing flooding and sedimentation as seen in modern freshwater marshes and wet meadows. In the riparian and lake-margin settings in which much of the Middle Devonian flora is found, wet meadows were likely common, as increasing stature, rooting, and floral partitioning allowed for some plants to adapt to seasonal inundation or exposure.

Sphenopsid-like plants are another important shrubby clade that emerged in the Late Devonian. Included among these plants are the Iridopterids (Stein et al., 1984). Calamitalean sphenopsids of the Carboniferous appear to have been adept particularly at colonizing disturbed environments, such as riparian wetlands susceptible to flooding and sedimentation (Scott, 1978; DiMi- chele and Phillips, 1985; Gastaldo, 1987; Pfefferkom et al., 2001). In modem coastal, lacustrine, and riverine marsh settings, some emergent, reed-like plants are simplified (reduced) as an adaptation for living in these disturbance-prone areas. Reed-like

Figure 2. Illustration of the Rhynie geothermal wetlands. Arthropods include the crustaceans (A) Lepidocaris and (B) Castracollis, (C) a euthy- carcinoid, (D) the partial remains of centipede, (E) the trigonotarbid Palaeocharinus; and (F) the partial remains of a springtail. Flora include (G) Aglaophyton, (H) Rhynia, (I) Horneophyton, and (J) Asteroxylon. All floral members drawn to same scale. Illustrations based on data and reconstructions from the University of Aberdeen, Scotland (www.abdn.ac.uk/rhynie/).

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Evolution and importance of wetlands in earth history morphologies limit damage from storms and flooding through

reduction of surface area, and clonal growth allows reestablish- ment of aerial shoots if the emergent parts of the plants should be broken (Keddy, 2000) or buried (Gastaldo, 1992). Sphenopsid reed-like morphologies in disturbance-prone Carboniferous environments created a framework similar to that presently created by reeds and rushes. Thick stands of reeds in modern marshes serve important functions in terms of sedimentation control, water filtering, flood control, and habitat, all of which are likely to have originated in Devonian marshes.

The Oldest Swamps

During the Middle to Late Devonian, lycopsids and progymnosperms attained tree-like stature, which led to the evolution of the first true forested wetlands, by definition, swamps (Fig. 1).

Lycopsids were the first land plants to develop shallow substrate- penetrating roots (Remy and Remy, 1980), which advanced the process of soil development. Other clades evolved root systems later in the Devonian (Driese and Mora, 2001), altering pedogenic processes. Root systems were essential to the development of an arborescent growth habit because of the centralized growth form of most trees. Arborescence continued the pattern of increasing vegetational zonation, with the development of tiered canopies, including both trees and understory shrubs (Scott, 1980). Zona- tion contributed directly to the differentiation of swamps and marshes and the development of new niche space (Scheckler,

1986a; Cressler, this volume), and thereby biodiversity.

What may be the oldest swamps (forested wetlands) were reported by Driese et al. (1997) from the Middle Devonian of New York. Large stumps and shallow-penetrating roots, attrib- utable to cf. Eospermatopteris, are preserved in a gray-green, gleyed, pyritic mudstone, interpreted as a waterlogged paleosol.

Bartholomew and Brett (2003) have redescribed similar in situ stumps of Eospermatopteris (possibly a cladoxylalean) from the famous Gilboa locality in New York, from which the genus was described originally (Goldring, 1924). Although the habit of this plant is uncertain, stumps of approximately one meter in diam- eter have been reported, suggesting large trees adapted to wetland (swamp) conditions.

The progymnosperm Archaeopteris sp. is considered the oldest typically woody, tall tree (Figs. 1, 3), growing to heights of 18m and occupying poorly drained flood plains and coastal areas (Beck, 1962,1964; Retallack, 1985; Scheckler, 1986; Meyer-Ber- thaud et al., 1999). As such, they formed true gallery forests in floodplain environments constituting riverine or paludal forested wetlands, riparian forest-wetlands, or swamps (when defined as forested wetlands on mineral substrates). Arborescent progymnosperms had flattened branch systems and leaves, providing for a canopy and the potential for a shaded understory, which, in the Late Devonian, was dominated by the scrambling fern-like plant Rhacophyton (Fig. 3). In combination, this plant association would have increased litter input to the swamp floor (DiMi- chele and Hook, 1992; Algeo et al., 1995; Algeo and Scheckler,

1998), providing increased nutrients to surrounding wetland, fluvial, and upland ecosystems. The result of litterfall detritus in extant wetlands is the formation of a complex detritus-based food web that supports a great diversity of aquatic invertebrates, fish, and amphibians, often with greater biodiversity than in adjacent uplands because of the "edge effects" of ecotones (Bacon, 1997;

Keddy, 2000; Mitsch and Gosselink, 2000). Such a food web was likely in place by the Middle Devonian.

The development of deep, extensive roots in Frasnian progymnosperms resulted in increased substrate stabilization (Figs. 1, 3) and a change in the rate at which paleosols formed and sediment was discharged (Algeo and Scheckler, 1998; Algeo et al., 2001). Devonian substrate stabilization also decreased sediment fluxes and reduced catastrophic flooding in wetland habitats (Schumm, 1968; Beerbower et al., 1992). This latter consequence

Figure 3. Devonian lacustrine wetland dominated by the pre-fem Rha- cophyton and the progymnosperm Archaeopteris, whose roots stabi- lize the banks of the oxbow lake.

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S.F. Greb, W.A. DiMichele, and R.A. Gastaldo is an important function of modem wetlands, where flooding is

prevented through the "breaking" action supplied by thick stands of plants against floodwater velocity, as well as through floodwater storage (Mitsch and Gosselink, 2000; Keddy, 2000). It also could lead to reduced runoff and increased precipitation, leading to significant changes in the global hydrological cycle (Algeo and Scheckler, 1998; Algeo et al., 2001).

Roots are central in the process of denitrification, which is important in global nitrogen cycling (e.g., Keddy, 2000). This critical function presumably originated in mid-Devonian marshes but increased with the evolution and spread of true swamps, and the development of upland forests leading to a dramatic increase in vegetative primary productivity. These expansions across the landscape would have increased carbon consumption and atmo- spheric carbon dioxide (pCO^) drawdown. In combination with

Figure 4. Devonian mires were dominated by the pre-fem Rhacophy- ton, but arborescent lycopods with stigmarian roots became increasingly common.

increased nutrient flux and bottom water anoxia and organic carbon fluxes, these perturbations could have led to global cooling, Devonian glaciation, as well as the end-Devonian mass extinction (Berner, 1993, 1997; Algeo et al., 1995; Algeo and Scheck- ler, 1998).

The Oldest Mires

Late Devonian coals record the evolution of the first peat- accumulating wetlands, indicating when plants had evolved the production and shedding of prolific amounts of biomass, which allowed peat to accumulate under specific chemical conditions. There is a distinction made between modem peat and non-peat-forming wetlands in most discussions (Mitsch and Gosselink, 2000; Keddy, 2000), and many authors differentiate between swamps and mires (bogs, fens; e.g.. Gore, 1983). Peats are composed of at least 50% organic (mostly plant) material and accumulate where organic production outpaces decomposi- tion, generally in wet, low-oxygen substrates. Often, the presence of an impervious aquiclude underlying the peat mire allows for the stilting of the water table, promoting litter accumulation (Gastaldo and Staub, 1999). Peat substrates present plants with considerably different challenges than mineral substrates. Most importantly, many peats are relatively nutrient deficient because organic matter chelates mineral nutrients. Stability for rooting also differs from mineral substrates. Finally, pore waters in peat, in some cases, have a lower pH than what most plants experience on other types of substrates (DiMichele et al., 1987; Cross and Phillips, 1990; Gastaldo and Staub, 1999). Peat accumulation in the Devonian resulted in new types of wetlands and new wetland functions associated with mires.

Some of the earliest coals are interpreted as sapropelic

"boghead" coals, which form from the accumulation of algae in brackish to freshwater restricted environments (Thiessen, 1925;

Sanders, 1968), although most result from the accumulation of terrestrial detritus. Several Late Devonian (Frasnian) coals of eastern North America are dominated by the herbaceous scrambling fern Rhacophyton (Scheckler, 1986a; Cross and Phillips, 1990). These sites would be classified as shrub-dominated peat wetlands or "fens" (Fig. 1; Gore, 1983; Keddy, 2000; Mitsch and Gosselink, 2000). Because Rhacophyton grew in both mineral and peat substrates, it likely was preadapted to oligotrophic conditions, which allowed this marsh plant to become one of the initial mire creators/occupiers.

Forested mires also appear in the Late Devonian and are composed of lycopsids (Figs. 1,4). Late Devonian coals of China are dominated by the arborescent lycopsids Lepidodendrop- sis, Lepidosigillaria, and Cyclostigma (Xingxue and Xiuyhan, 1996). Arborescent lycopsids originated in non-peat-accumulating Devonian swamps and later expanded their range into peatlands, where they became dominant. It has been inferred that as peatlands expanded, these ecosystems became refugia for relict plants (like the lycopsids), as increasing morphological innova- tion allowed other clades to expand outside of wetland habitats

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Evolution and importance of wetlands in earth history (Knoll, 1985; DiMichele et al., 1987). The stigmarian root sys-

tems of lycopods (Fig. 4) permitted growth in wet, oxygen-poor, soft-sediment substrates (Rothwell, 1984; DiMichele and Phil- lips, 1985; Phillips et al., 1985) and allowed lycopods to become the dominant vegetation of the Carboniferous peatlands.

Late Devonian forested mires may represent the earliest bogs, depending on the use of the term. Bogs generally are differ- entiated from fens by the accumulation of thicker peat composed of vegetation that is at least partly arborescent. In this respect.

Late Devonian forest mires could be termed bogs. Devonian forest mires, however, were not ombrotrophic or dominated by mosses, characteristics implied in some uses of "bog" (Mitsch and Gosselink, 2000; Keddy, 2000). In terms of their ecological functions, these Devonian fens and forest mires mark the initiation of a new carbon sink, contributing to changes in the global carbon cycle and remaining important to this day. Also, the high water-storage capacity of peats means that mires can significantly influence local and regional hydrology (Mitsch and Gosselink, 2000; Keddy, 2000), which likely began in the Devo- nian but would have greater impact with the spread of mires in the Carboniferous.

Tetrapod Evolution and Wetlands

Tetrapods made landfall in the Late Devonian (Milner et al., 1986; Clack, 2002) from lungfish and lobe-finned fish ancestors.

In fact, low-oxygen conditions caused by decaying plant matter in freshwater wetlands and wetland-fringing lakes may have spurred the evolution of tetrapod lungs (Randall et al., 1981; Car- roll, 1988; Clack, 2002). Extant lungfish, such as the Australian Neoceratodos forsteri and African Protopterus, inhabit freshwater rivers, ponds, and marshes. They survive in ephemeral wetlands by burrowing into and estivating within wet substrates, sur- viving for many months until seasonal rains reflood their habitat (Speight and Blackith, 1983).

Acanthostega is one of the earliest aquatic tetrapods. Its mul- tidigit appendages were preadapted for use on land, having first evolved in water (Gould, 1991; Clack, 1997; Clack and Coates 1995; Coates and Clack 1995). In the fluvial environments in which Acanthostega is preserved, it has been hypothesized that digitation was useful in strong currents for grasping onto rocks and water plants (Clack, 1997). Terrestrial mobility may have originated as a preadaptation in these earliest tetrapods that developed in association with maneuvering through vegetation in fluvial (riparian) wetlands dominated by dense stands of Rha- cophyton in Late Devonian riverine marshes (Fig. 5).

Amphibians are common in many modem riverine/riparian wetlands (Mitsch and Gosselink, 2000) and many extant species require this habitat for part of their life cycle. Modem amphibian distribution is influenced by predation and the stability, light intensity, and temperature of their habitats (Skelly et al., 1999, 2002). Broad wetlands, with distinct microhabitats of overstory, midstory, and shrub, provide different types of food and cover where amphibians generally are abundant (Rudolph and Dick-

son, 1990). By the Late Devonian, tiering and canopy zonation in marshes, swamps, fens, and forest mires was well established, and created the types of food and cover in which tetrapods could thrive, adding another layer to both freshwater aquatic and terrestrial food webs.

MISSISSIPPIAN

Tetrapod and Wetland Diversification

Tetrapods continued to evolve and diversify into the Car- boniferous as exemplified by one of the most famous Lower Carboniferous sites in East Kirkton, England. The fossil-bearing limestone preserves a wide variety of vertebrates, including chondrichthyan and acanthodian fish, lungfish, temnospondyls, anthracosaurs, and a reptiliomorph (reptile-like) animal (Milner and Sequeira, 1994). At one time, the reptiliomorph nicknamed

"Lizzie" was interpreted as the oldest amniote (reptile; Smithson, 1989). More recent studies, however, have suggested that it was only a close relative of amniotes (Smithson et al., 1994), and possibly even a stem-tetrapod or an early amphibian, rather than a true amniote (Laurin and Reisz, 1999).

The East Kirkton tetrapod assemblage occurs in an alkaline, freshwater lake rimmed with marshes formed from reed-like calamites and a pteridosperm with Sphenopteris foliage (Milner et al., 1986). Volcanogenic rocks preserve several different plant assemblages within hydrothermal hot-spring deposits (Rolfe et al., 1990; Brown et al., 1994; Scott and Rex, 1987; Galtier and Scott, 1994; Scott et al., 1994). The vertical juxtaposition of these assemblages indicates that East Kirkton initially was a

Figure 5. Acanthostega maneuvers through stands of the pre-fern Rha- cophyton and roots of the arborescent progymnosperm Archaeopteris in a flooded Devonian riparian marsh.

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10 S.F. Greb, W.A. DiMichele, and R.A. Gastaldo lake surrounded by drier, pteridosperm-dominated woodlands;

these subsequently were altered to wetter substrates in which lycopod-dominated swamps are preserved (Scott et al., 1994).

Many Carboniferous tetrapod assemblages accumulated in similar open-water bodies fringed by marshes or forest swamps, and in swamp-filled pools (oxbows, billabongs) (Milner et al., 1986;

Hook and Baird, 1986; Garcia et al., this volume).

Tuffs containing fusain at East Kirkton may indicate that volcanic activity ignited wildfires (Brown et al., 1994), which in turn may have driven the vertebrates from this landscape into the lakes where they perished (Scott et al., 1994). Fires are important elements in the ecology of most modem wetlands and influence floral content and community succession in extant wetlands (Keddy, 2000). This probably has been the case since the Late Devonian (Scott, 1989).

Possible Mangal Wetland Origins

The Mississippian provides the first evidence for the expansion of any clade into nearshore and marginal marine sites, those under possible saline influence. Inasmuch as the term "mangrove"

often is applied to woody taxa, the term mangal—any salt-tolerant plant—would be applied to these assemblages. Gastaldo (1986) interpreted the stigmarian-rooted lycopsids reported by Pfefferkom (1972) in the Battleship Wash Formation, Arizona, as representing the first mangal taxon. Gastaldo et al. (this volume) also demonstrate that some Mississippian back-barrier marshes were inhabited by herbaceous, cormose lycopsids. Most arborescent lycopsids are interpreted to have been intolerant of salt water (DiMichele and Phillips, 1985), although smaller, cormose forms, such as Chaloneria, have been interpreted as living in coastal marsh-like habitats (DiMichele et al., 1979), as well as fresh-water marshes and peat-forest swamps (Pigg, 1992). It is not a simple proposition to identify morphological features that would support a brackish-habitat interpretation for Paleo- zoic plants because not all of these adaptations (for example stilt roots) are solely an adaptation to saline tolerance. Transgression (onlap) could result in burial of freshwater, near-coast taxa in marine sediments, confounding interpretations of mangal habit based on sedimentological evidence. In addition, many extant freshwater wetland plants and mangals live on freshwater lenses in the soil, adjacent to brackish or marine waters. Few plants can tolerate the precipitation of salts in marine-water influenced soils. Therefore, interpretation of mangal habit is, in part, a matter of recognizing that the plants did not live directly within fully marine salinities but could tolerate the incursion of salt water, or recognizing physiological features that allow an interpretation of salinity tolerance.

Spreading Mires and Lowland Swamps

Within the coastal plains and continental interiors, extensive swamps and thick peat mires first occur in the Late Mississippian throughout Eurasia including Canada, western Europe, Ukraine,

Belarus, Russia, and China (Wagner et al., 1983; Scotese, 2001;

Rygel et al., this volume). These are dominated by arborescent lycopsids that range throughout the late Early Carboniferous up to near the Mississippian-Pennsylvanian boundary, where they first are joined by typical Pennsylvanian lycopsid taxa. The lycopsids Lepidophloios and Paralycopodites remain a component of these mires into the Pennsylvanian, while new species of the lepi- dodendrid complex replace typical Early Carboniferous forms.

A few floristic elements of the Early Mississippian (Visean) per- sist into the Namurian mires in the Silesian basin, mostly within the sphenopsids {Archaeocalamites and Mesocalamites) and fem/pteridosperms (Purkynova, 1977; Havlena, 1961). Although much of the global Mississippian is recorded in carbonate ramp deposits, it is these lycopsid-dominant swamps and mires that set the stage for the extensive accumulation of peatlands in the Pennsylvanian.

PENNSYLVANIAN

The Heyday of Tropical Mires

Pennsylvanian (Upper Carboniferous) coals are known from basins in the eastern and central United States, eastern Canada, England, eastern and western Europe as well as parts of China and East Asia (Walker, 2000; Scotese, 2001; Thomas, 2002).

These areas straddled the Pennsylvanian equator (Fig. 6), with some coals representing the most widespread tropical mire systems in earth history (Greb et al., 2003).

Much is known about the ecology of Pennsylvanian wetland plants and plant communities, a consequence, in part, of expo- sures made possible by the mining of economically important coals. The ecologies of the dominant plant groups have been reviewed by DiMichele and Phillips (1994), but recent data from the Early Pennsylvanian (Langsettian) may indicate that the partitioning of ecospace within mires occurred through the Penn- sylvanian (Gastaldo et al., 2004). In brief, giant lycopsid trees were restricted mostly to wet, periodically flooded substrates.

These trees dominated Early and Middle Pennsylvanian forest mires. Lycopsids were spore producers, although some had seed- like "aquacarps," adapted for aquatic fertilization and dispersal in forested wetlands (Phillips and DiMichele, 1992). They were supported by bark, rather than wood, and had highly specialized rooting systems {Stigmarid) that facilitated growth in low-oxygen, soft substrates. There was a variety of lycopsid tree genera with specializations to different levels of disturbance and substrate exposure (Fig. 7A-7E).

Other spore-producing groups coexisted with the lycopsids in these mires. Marattialean tree ferns of the genus Psaronius were cheaply constructed plants (Baker and DiMichele, 1997);

tree habit was made possible by a thick mantle of adventitious roots (Figs. 7A, 7C, 7F). The calamiteans were another group, closely related to modern scouring rushes and horsetails of the genus Equisetum. Extant Equisetum is a small, widespread, non- woody plant that grows in moist places and poor soil. Calamite-

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Evolution and importance of wetlands in earth history 11 ans appear to have inhabited the same environments (Fig. 7A),

although some calamiteans grew to heights in excess of 5 m and, hence, would have served functions more similar to small trees than shrubs (Fig. 7D). The calamiteans were the only major Late Carboniferous tree group to exhibit clonal growth. Aerial stems developed from subterranean rhizomes in most species, a growth form that permitted them to exploit habitats with high rates of sediment aggradation (Fig. 7F) in which the stems could be buried repeatedly by flood-borne siliciclastics and continue to regen- erate (Gastaldo, 1992).

Two seed-producing tree groups also were common in peat- substrate mires, the cordaites (Fig. 7F) and the meduUosan pteridosperms (Figs. 7C, E). Cordaites were woody trees and shrubs closely related to extant conifers. In the middle Westphalian, cordaitean gymnosperms became abundant in some parts of mire landscapes (Fig. 7F), apparently reflecting areas with periodic extended substrate exposure or disturbance (Phillips et al., 1985). Cordaites were also common components of late Paleo- zoic Angaran (Asian) wetlands (Oshurkova, 1996). Some forms have been reconstructed as mangroves (Cridland 1964; Raymond and Phillips, 1983), although evidence of stilt-like roots is lacking in preserved Cordaites tree trunks (Johnson, 1999). It also has been suggested that they could tolerate brackish conditions (Wartmann, 1969). There is, however, substantial reason to doubt a mangrove interpretation, given that the plants appear to prefer rotted peat, possibly subject to exposure, and that they occur in a complex flora associated with an array of other plants that do not appear to be specifically adapted to salt-water tolerance (Phillips et al., 1985; Raymond et al., 2001).

MeduUosan pteridosperms were small trees largely confined to nutrient-enriched substrates. They produced large fronds on which were borne some of the largest seeds known among Car- boniferous tropical plants (Gastaldo and Matten, 1978). Medul- losans were free standing and formed thickets or tangles of plants that leaned on each other for support (Wnuk and Pfefferkom, 1984). In addition to these tree forms, representatives within the pteridosperms, ferns, sphenopsids, and lycopsids also displayed ground cover and liana (vine) growth strategies (Fig. 7B-7D, 7F).

A liana growth strategy is important in modem tropical wetlands because it allows plants to compete for light amongst tall trees.

These forms were systematically diverse and occasionally abundant in the community (Hamer and Rothwell, 1988; DiMichele and Phillips, 1996a; DiMichele and Phillips, 2002).

Non-Peat-Forming Swamps

Non-peat-forming swamps, sometimes referred to as "clastic swamps" (e.g., Gastaldo, 1987; Mapes and Gastaldo, 1986;

Gastaldo et al., 1995) also were widespread in Pennsylvanian coal basins. These habitats supported a vegetation much like that of forest mires, although there were many species-level differences and the environments were dominated by different plants.

Swamp habitats often were enriched in lycopsids but included pteridosperms as major components (Wnuk and Pfefferkom,

Cold

Figure 6. Upper Carboniferous paleogeography and climates showing locations of coal (black dots) and thereby known paleomires (modified from Scotese, 2001). AU = Australia, CH = China, EU = Europe, NA

= North America, SA = South America, SI = Siberia.

1984; Scott, 1978; CoUinson and Scott, 1987; Gastaldo, 1987).

In the late Middle Pennsylvanian, marattialean tree fems began to increase in abundance in all parts of the wetland landscapes, although the increase in fem abundance can be detected in clastic substrate habitats (marshes and swamps) before it appears in mire habitats (Pfefferkom and Thomson, 1982). A major extinction at the end of the Middle Pennsylvanian (Westphalian) resulted in a significant reorganization of wetland ecology (Phillips et al., 1974; DiMichele and Phillips, 1996b). Following the extinction, Euramerican Late Pennsylvanian (Stephanian) mire and riparian swamps became more similar in overall patterns of dominance and diversity. Psaronius tree ferns were dominant, meduUosan pteridosperms were subdominant, and Sigillaria (Fig. 7D), a tree lycopsid that may have preferred periodic substrate dryness, was locally common.

The Development of Wetland Successions

Many modem peatlands exhibit a temporal succession of wetland types in response to changing hydrology and nutrients (Gore, 1983; Moore, 1989; Mitsch and Gosselink, 2000; Keddy, 2000).

The earliest definitive successions in ancient weflands are from the Pennsylvanian. Studies of English Pennsylvanian coals by Smith (1957, 1962) noted that many exhibit vertical changes in spore content, which were inferred to represent changes or successions in plant (and wetland) types. Coals also exhibit vertical changes in ash yield, sulfur content, palynology, and petrography, which result from temporal succession of different wetland types (Cecil et al., 1985; Esterle and Ferm, 1986; Eble and Grady, 1990; Greb et al., 1999a, 2002). Successional patterns also have been inferred from coal balls (Raymond, 1988; Pryor, 1993; Greb et al., 1999b).

Many Euramerican coals began in topographic lows as marshes or swamps (Figs. 7A) and then became mires as conditions allowed for the unintermpted accumulation of biomass. In some cases, successions occurred between different mire types.

Thin peats, rooted seat earths (poorly developed soils—e.g., incep-

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Figure 7. Pennsylvanian wetlands were diverse and included (A) pioneering topogenous riverine and paludal mires and swamps, (B) flooded swamps and topogenous forest mires, (C) paludal swamps, (D) riverine/riparian-margin marshes and swamps, and (E) ombrogenous mires.

Disturbance-prone mires (F) along wetland margins were dominated by disturbance-tolerant flora. Swamps and forest mires were dominated by lycopods including Paralycopodites (Lp), Lepidophloios (Lls), Lepidodendron (Lin), Sigillaria (Ls), and Omphalophloios (Lo). Juvenile Lepidodendron (Llj). Lycopod reconstructions based on DiMichele and Phillips (1985, 1994). Other arborescent flora included tree ferns (Tf), sphenopsids such as Catamites (Ca, which ranged from herbaceous to arborescent), and the gymnospermous tree Cordaites (Co). Sphenopsids also occurred as vines (lianas). Ground cover was dominated by ferns and sphenopsids.

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Evolution and importance of wetlands in earth history 13 tisols or entisols), and in situ tree stumps within mineral substrates

form in a wide variety of swamp and marsh settings (e.g., Tei- chmiiller, 1990); thick peats can accumulate in fens and forested mires. Extant planar peatlands, also called topogenous mires or low-lying moors, generally occur at or just below the ground-water table and tend to fill in the topography (Gore, 1983; Moore, 1989).

Ombrogenous mires, or raised mires in which peat doming may occur, build up above the topography in everwet climates (Gore, 1983; Clymo, 1987; Moore, 1989). Successions from topogenous to ombrogenous mires have been inferred for numerous Eurameri- can Pennsylvanian coals on the basis of palynological analyses (Cecil et al., 1985; Esterle and Ferm, 1986; Eble and Grady, 1990;

Grab et al., 1999a, 1999b). Because modem ombrogenous mires build up above surrounding river levels, they are low-nutrient habitats without standing water cover and are dominated by stunted vegetation in the domed areas (Gore, 1983; Moore, 1989). In the Pennsylvanian, similar conditions are inferred for ombrogenous mires, which appear to have been dominated by stunted lycopsids (Omphalophloios) and ferns (Fig. 7E; Esterle and Ferm, 1986;

Eble and Grady, 1990; Greb et al., 1999a, 1999b).

Figure 8. Giant arthropods in Pennsylvanian wetlands included the millipede Arthropleura and giant mayflies (lower right) here shown in a lycopod swamp.

Giant Arthropods in Wetlands

The record of Carboniferous arthropods is very good, partly because of the many Carboniferous concretion locations that are fossiliferous, including the famous Mazon Creek area of the Illinois Basin and Montceau-les-Mines, France (Darrah, 1969;

Gastaldo, 1977; Nitecki, 1979; Baird et al., 1986). Much of the primary plant productivity in Late Carboniferous wetlands continued to reach animal food webs through arthropod detritivores, although a relatively complete trophic web of detritivores, herbivores, and carnivores had developed (DiMichele and Hook, 1992; Labandeira and Eble, 2006). Arthropleura was a giant millipede-like arthropod (Fig. 8) that consumed the inside of rotting lycopod trunks on swamp and forest-mire floors (Rolfe, 1980; Hahn et al., 1986; Scott et al., 1992). At 1.8 m in length, Arthropleura is the largest terrestrial arthropod of all time (Rolfe, 1985). Their large size suggests that arthropleurids filled a niche that had yet to be shared with tetrapods (DiMichele and Hook, 1992), or that tetrapods were not yet large enough to pose a threat. Millipedes are still important wetland detritivores but are much smaller than Arthropleura. Cockroaches are another common extant detritivore and were particularly abundant in Carboniferous wetlands (Durden, 1969; Scott et al., 1992;

Easterday, 2003), reaching 8 cm in length.

In addition to their importance as litter-dwelling wetland detritivores, some Carboniferous arthropods also evolved flight (Kukalova-Peck, 1978, 1983: Scott et al., 1992; Labandeira and Eble, 2006). One explanation for the origin of flight is that wings evolved from gills in aquatic stages, and flight evolved through surface-skimming, a process used by extant, wetland-inhabiting stone flies (Plecoptera) and subadult mayflies (Ephemeroptera;

Marden and Kramer, 1994). Giant mayflies with wingspans of more than 40 cm are known from Late Carboniferous wetland facies (Fig. 8; Kukalova-Peck, 1983). The most commonly

depicted flying insect in Carboniferous illustrations is Meganeura, a dragonfly-like hexapod, which had a wingspan of more than 60 cm. The precursors of extant dragonflies, the Protodonata, also evolved in the Carboniferous, and some had wingspans of more than 60 cm (Carpenter, 1960). Extant dragonflies are common predators of wetlands. Because most dragonflies have aquatic nymphs, they require wet habitats for part of their life cycle. In fact, the evolution of metamorphosis in insects appears to have occurred in wetland or wetland-fringing ecosystems (Kukalova- Peck, 1983; Truman and Riddiford, 1999).

Insect flight also may have contributed to the rise of insect herbivory, as flying insects could exploit new food resources (DiMichele and Hook, 1992). Some Carboniferous insects (e.g., megasecopterans and paleodictyopterans) developed mouth parts for sucking and piercing. Evidence for this strategy is found in permineralized swamp-and-mire plants (Scott et al., 1992;

Labandeira and Phillips, 1996; Labandeira and Eble, 2006). In fact, most major insect herbivore functional feeding groups on land were established by the late Paleozoic and are preserved in wetland and wetland-fringing estuarine and lacustrine sediments (Labandeira and Eble, 2006). Insect herbivory brought the wetland food web closer to modem trophic systems.

Amniote Evolution and Wetlands

The oldest undisputed amniote, the "protorothyridid" Hylo- nomus from the Middle Pennsylvanian of Joggins, Nova Sco- tia (Dawson, 1854; CarroU, 1964; DiMichele and Hook, 1992;

Calder, et al., 1997; Calder et al., this volume), appears to be a very early member of the lineage that led to diapsids. Although reptiles do not require aqueous conditions for breeding, as do amphibians, many do require wetlands for food and cover (Fig. 9;

Clark, 1979). At present, reptile abundance is influenced by the

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14 S.F. Greb, W.A. DiMichele, and R.A. Gastaldo availability of horizontal and vertical habitat (Jones, 1986), as

may have been the case in the Carboniferous. In the layered canopies of Pennsylvanian peatlands and forest swamps, there was abundant habitat availability for food and cover. At Joggins, reptiles were found within fossil hollowed lycopsid tree stumps;

Dawson (1854) originally thought that the animals had fallen into the stumps and been trapped. More recent investigations interpreted the stumps as possible dens in which the reptiles died during wildfires (Calder et al., 1997; Falcon-Lang, 1999). This interpretation is plausible, given that modem wetlands are susceptible to seasonal wildfires, especially crown fires (Scott, 2001).

PERMIAN

High-Latitude Peatlands in Gondwana

Although Permian coals are sometimes considered part of the first great coal-forming period (Permo-Carboniferous), most, with the exception of some coals from the Permian of China (Xingxue

Figure 9. Hylonomus, one of the first reptiles, takes shelter in a hol- low lycopod trunk during a Pennsylvanian swamp fire in what is now Nova Scotia.

and Xiuyhan, 1996), are geographically and floristically separate from their Carboniferous precursors. Pennsylvanian coals represent mostly tropical to subtropical mires that were widespread in Euramerican basins. By the Early Permian, North American coals were restricted to the northern Appalachian Basin, and these mires represented a holdover of Pennsylvanian floras into the Permian.

Tropical coals became restricted to several Asian plates (Scotese, 2001). The most widespread peatlands flourished in the cool- temperate climates of the southern Gondwana supercontinent (Fig. 10). These included the first evidence of peats accumulating under permafrost conditions, similar to modem palsa mires (KruU, 1999). Some of these high-latitude Gondwana mires were the first extensive, nontropical mires in earth history and data sug- gest that there was latitudinal plant zonation (toward both poles), analogous to the modem latitudinal gradients in Northem Hemi- sphere wetlands (Retallack, 1980; Archangelski, 1986; Cuneo, 1996; Xingxue and Xiuyhan, 1996).

The majority of coal resources in present-day Australia, India, South Africa, and Antarctica are of Permo-Triassic age (Archangelsky, 1986; Walker, 2000; Thomas, 2002). The floral composition of Gondwana coals is distinctly different from the Carboniferous coals of the Northern Hemisphere. Whereas Car- boniferous mires were dominated by lycopods and tree ferns, Permian Gondwana mires were dominated by gymnosperms (Archangelski, 1986; Falcon, 1989; Cross and PhiUips, 1990;

Shearer et al., 1995). In the Early Permian, Gangamopteris was dominant. By the Middle Permian, Glossopteris was dominant.

Many species are interpreted to have had both herbaceous and arborescent growth strategies (Falcon, 1989; Taylor and Taylor, 1990; White, 1990; Stewart and RothweU, 1993; Shearer et al., 1995). Arborescent Glossopteris taxa were tall, with Dadoxylon- Araucarioxylon-type gymnospermous wood and Vertebraria- type roots (Fig. 11, Gould and Delevoryas, 1977; Stewart and RothweU, 1993). The arrangement of secondary xylem and the presence of large air chambers in the roots indicate that these trees were adapted to standing water or waterlogged soils in swamp and forest mire settings (Gould, 1975; Retallack and Dilcher, 1981; White, 1990). The similarities between Glossopteris taxa on different Southem Hemisphere continents, and the recognition that Glossopteris-rich, coal-bearing strata accumulated under different climatic conditions from those of today, were some of the original data used to support the theory of continental drift.

Glossopteris mires also were composed of abundant horsetails, fems, herbaceous lycopsids, and bryophytes (Neuburg, 1958; Archangelski, 1986; White, 1990) in a wide array of wetland types, including algal ponds, reed fens dominated by the sphenopsid Phyllotheca, wet forest mires, and dry swamp forests (Diessel, 1982). The association of bryophytes with high-latitude mires continues to this day in the world's most widespread peatlands, the Sphagnum-AovmnaitA peats of West Siberia (Botch and Masing, 1983) and the Hudson Bay Lowlands (Zoltai and PoUett, 1983). Another wetland association that began in the Permian was that with large semiaquatic vertebrates. Today alli- gators, crocodiles, and gavials play a similar ecological role. In

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Evolution and importance of wetlands in earth history 15 Cold

Gondwana-

Figure 10. Permian paleogeography and paleoclimates showing locations of coal (black dots) and thereby known paleomires (modified from Scotese, 2001). AF = Africa, AN = Antarctica, AU = Austraha, CH = China, CI = Cimmeria, IN = India, SI = Siberia

the Permian, (and Late Carboniferous) large, semi-aquatic laby- rinthodont temnospondyls were found in these roles. The 1.8-m long Eryops is one of the most common and widespread early Permian labyrinthodonts (Carroll, 1988). Later in the Permian, the rhinesuchids evolved from the eryptoid labyrinthodonts.

Rhinesuchids had elongated skulls with eyes on top of their skull similar to extant crocodilians (Fig. 11).

Climatic Changes and Shrinking Wetlands

At the same time that the northern and southern continents were amalgamating to Pangaea, the late Paleozoic ice age was ending, with the last vestiges of Southern Hemisphere ice disap- pearing in the earliest Permian (Frakes et al., 1992). The termina- tion of ice-age climates, and the sea-level periodicity associated with them, led to an overall climatic warming, which resulted in drying and a dramatic decrease in the scale and extent of wetlands when compared with the Carboniferous. Under these new conditions, some of the previously dominant spore-producing plants were restricted to narrow riparian corridors and lakeside settings (DiMichele and Chancy, this volume). The exception to this pattern occurs on the Chinese microcontinents, which remained climatically wet owing to their proximity to oceanic moisture sources. This region maintained wetland floras similar to those of the Middle Pennsylvanian (lycopsids, cordaites, tree ferns); such floras persisted into the Late Permian (Xingxue and Xiuyhan, 1996; Rees et al., 2002).

Changing climates and flora resulted in distinct global floristic zones (Ziegler, 1990; DiMichele and Hook, 1992; Rees et al., 2002). Today, latitudinal climate distribution results in zonation of different types of wetlands (e.g., extensive Sphagnum bogs at high latitudes, marshes in the temperate zone, and mangrove swamps in the Neotropics). Middle to Late Permian coals of the Southern Hemisphere are dominated by wood and leaves of the

Figure 11. During the Permian, forest mires spread across Gondwa- naland with gymnosperms replacing lycopods as the dominate wetland trees. The mire is dominated by Glossopteris trees and a ground cover of ferns and horsetails. Rhinesuchus (in the water) watches a dicynodont on shore. Glossopteris tree reconstruction after Gould and Delevoryas (1977).

pteridosperm Glossopteris (Fig. 11), whereas coeval peats in Siberia are composed of biomass from ruflorian and voynovsky- alean cordaites (e.g., Meyen, 1982; Taylor and Taylor, 1990).

Ziegler (1990) discusses latitudinal zonation of Permian biomes.

Regional Permian drying resulted in the diversification of seed plants, with the evolution and diversification of ginkgophytes, cycads, peltasperms, and filicalean ferns.

Just as the loss of wetland habitats perturb modern ecosystems, the loss of Permian wetlands had profound influences on terrestrial ecosystems at the close of the Paleozoic. In the Karoo Basin of South Africa, where the most complete terrestrial record occurs across the P-Tr boundary, there is a basinward shift from riparian wetlands to dry uplands through the Permian. This shift